If an item within a project is marked as red it is unavailable during the time of the project.
In the main material tab and in a project you can quickly check the availability of items by selecting them and clicking on "Check availability".
The color legenda can be opened by clicking on the question mark which is shown in the material availability window. By doing that you can see which items are available and which have already been rented out. Furthermore it is stated in which projects the unavailable materials are being used at the moment.
Counting projects in option
You have the possibility to select if you want include projects that have not yet been confirmed in the availability calculation.
Sum of the timeline
Per item and time period you are able to see how many items are available. To see this information move your courser on the desired time period in the availability window.